Multilayer optical films are known. Such films can incorporate a large number of thin layers of different light transmissive materials, the layers being referred to as microlayers because they are thin enough so that the reflection and transmission characteristics of the optical film in or near the visible spectrum are determined in large part by constructive and destructive interference of light reflected from the layer interfaces. Depending on the amount of birefringence (if any) exhibited by the individual microlayers, and the relative refractive index differences for adjacent microlayers, and also on other design characteristics, the multilayer optical films can be made to have reflection and transmission properties that may be characterized as a reflective polarizer in some cases, and as a mirror in other cases, for example. Whether the reflective characteristic is a polarizer or mirror, it is also known to select the thicknesses of the microlayers so that reflections occur in a desired part of the electromagnetic spectrum, e.g., in the visible or near infrared portion of the spectrum, or in portions thereof.
Some multilayer optical films are designed for narrow band operation, i.e., over a narrow range of wavelengths, while others are designed for use over a broad wavelength range such as substantially the entire visible or photopic spectrum, or the visible or photopic wavelength range together with near infrared wavelengths, for example. In a broadband reflector, the microlayers are arranged in optical repeat units whose optical thickness values increase along a thickness axis from a first side to a second side of the film. This arrangement of layer thicknesses is referred to as a graded layer thickness profile.
Researchers from 3M Company have pointed out the significance of layer-to-layer refractive index characteristics of such films along the direction perpendicular to the film, i.e., the z-axis, and shown how these characteristics play an important role in the reflectivity and transmission of the films at oblique angles of incidence. See, e.g., U.S. Pat. No. 5,882,774 (Jonza et al.). Jonza et al. teach, among other things, how a z-axis mismatch in refractive index between adjacent microlayers, more briefly termed the z-index mismatch or Δnz, can be tailored to allow the construction of multilayer stacks for which the Brewster angle—the angle at which reflectance of p-polarized light at an interface goes to zero—is very large or is nonexistent. This in turn allows for the construction of multilayer mirrors and polarizers whose interfacial reflectivity for p-polarized light decreases slowly with increasing angle of incidence, or is independent of angle of incidence, or increases with angle of incidence away from the normal direction. As a result, multilayer films having high reflectivity for both s- and p-polarized light for any incident direction in the case of mirrors, and for the selected direction in the case of polarizers, over a wide bandwidth, can be achieved.
Microlayers that are birefringent can be used in mirrors, polarizers, and other multilayer optical films. Researchers from 3M Company have recently disclosed techniques in which the reflective characteristic of such a film can be pattern-wise changed by exposing the film to a suitable light beam, where energy from the light beam is used to absorptively heat birefringent microlayers sufficiently to produce a relaxation in the material that reduces or eliminates a preexisting optical birefringence, but low enough to maintain the layer structure of at least most of the affected microlayers within the film. The reduction in birefringence may be partial or it may be complete, in which case some of the interior microlayers that are birefringent in a first (untreated) zone are rendered optically isotropic in a second (treated) zone. The selective heating may be achieved at least in part by selective delivery of light or other radiant energy to the second zone of the film. See, e.g., patent application publications WO 2010/075357 (Merrill et al.), WO 2010/075340 (Merrill et al.), WO 2010/075373 (Merrill et al.), WO 2010/075363 (Merrill et al.), and WO 2010/075383 (Merrill et al.).